It is standard practice to use large metal storage tanks for storing a variety of liquids, such as beverage and petroleum products. Conventional large storage tanks are usually made from non-stainless steel plates, and in the case of petroleum products the storage tanks are generally made from ¼ inch (0.63 cm) to ½ inch (1.27 cm) thick steel plates welded together. The dimensions of conventional large storage tanks usually range in the hundreds of feet (100 feet=30.5 meters) in height and hundreds of feet in diameter.
Non-contact electromagnetic detection and sensing may be used to determine the presence or signatures (object classification or shape) of objects, or liquid levels, or the distance to the surface of materials, when other sensing methods have difficulty in providing reliable and/or accurate information. For example, in the oil and gas industry, inaccurate or unreliable tank level measurements can incur a significant loss of profitability/revenue in tank level gauging applications. An error of 1 millimeter (mm) of the level measurement of a material in bulk storage tanks (40-80 meters in diameter) can correspond to volumetric error of several cubic meters. In the case of oil, since the crude oil price is generally at least $100 per barrel (1 barrel=42 US gallons; 159 liters), the 1-mm error can result in thousands of dollars loss for one or more parties involved in trading and oil transfer.
RAdio Detection And Ranging (Radar) has been used as a type of non-contact product level gauge for several decades. The radar system includes a transmitter coupled to a radar antenna which is positioned above the product (e.g., a liquid or solid) for emitting radar signals to the product and a receiver coupled to the antenna (or to another antenna) for receiving radar signals reflected from the product surface, as well as a signal processor for determining the product level on the basis of the emitted radar signals and the reflected radar signals. According to this method, the antenna driven by transmit circuitry emits a radar signal which strikes an object or surface, for example a liquid surface. The object or surface reflects part of the emitted radar signal/wave back in the direction of the antenna, which receives and is coupled to receive circuitry that processes the reflected radar signal/wave.
Pulse radars as a non-destructive/non-contact sensor for liquid level measurements in a storage tank are widely used, because they are relatively inexpensive and simple to install. One type of pulse radar system is Ultra-WideBand (UWB) radar. UWB radar systems transmit signals across a much wider frequency range as compared to conventional narrow-band pulse radar systems. The transmitted UWB signal is significant for its very light power spectrum, which is typically lower than the allowed unintentional radiated emissions for electronics. The most common technique for generating a UWB signal is to transmit pulses with very short pulse durations (e.g., ≦1 ns). The UWB pulse covers a very large frequency spectrum, and the frequency spectrum becomes larger as the pulse becomes narrower.
However, limited dynamic range, measuring distance and/or accuracy are drawbacks of pulsed radar including UWB radar in comparison with more complex signal-based radar including frequency-modulated continuous-wave (FMCW; which uses a continuous (linear) frequency sweep versus time) and stepped-frequency continuous-wave (SFCW; or frequency hopping) radar which are more expensive and complex. For UWB radar, the ultra short pulse width enables measurement accuracy, but results in shortening of the measuring distance due attenuation of the low pulse power.